Improving the Efficiency of Red Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diode by Rational Isomer Engineering

Abstract

The development of efficient red thermally activated delayed fluorescence (TADF) emitters with an emission wavelength beyond 600 nm remains a great challenge for organic light-emitting diodes (OLEDs). Herein, two pairs of isomers are designed and synthesized by attaching electron-donor 9,9-diphenyl-9,10-dihydroacridine (DPAC) moiety to the different positions of two kinds of highly rigid planar acceptor cores (PDCN and PPDCN). Their TADF efficiencies and emission maxima (599-726 nm) are regulated by molecular isomer manipulation. Interestingly, the photoluminescence quantum yields (Φ PL s) of trans-isomers T-DA-1 and T-DA-2 (78% and 89%) are remarkably higher than those of their corresponding cis-isomers C-DA-1 and C-DA-2 (12% and 14%). Significantly increased Φ PL values can be explained by single crystal structures and theoretical simulation. As a result, a deep red TADF-OLED based on T-DA-2 displays a maximum external quantum efficiency (EQE) of 26.26% at 640 nm. Notably, at a brightness of 100 cd m −2 , the EQE value of T-DA-2-based device still remains at an extremely high level of 23.95%, representing the highest value for reported red TADF-OLEDs at the same brightness. These results provide a reasonable pathway to optimize optoelectronic properties and thereby construct efficient red TADF emitters through rational isomer engineering.

A New Conjugated Polymer that Enables the Integration of Photovoltaic and Light‐Emitting Functions in One Device

“…Simultaneously, high charge transport in OPV cells was determined to be unfavorable to realizing high current efficiency (CE) and power efficiency (PE) in OLED devices because of the relatively low ability to confine charges to recombine. Accordingly, the CE and PE of the PBQx‐TCl:BTA3‐based device were determined to be not as outstanding as state‐of‐the‐art OLEDs, [ 22 ] but the EQE values are still 4 times higher than the early stage OLEDs based on polymers (Figure 4c). [ 23 ] As such, we are optimistic that the OLED performance of the device can be improved after being further optimized, without significantly sacrificing the OPV performance.…”

Twisted Biphenyl‐Diimide Derivatives with Aggregation‐Induced Emission and Thermally Activated Delayed Fluorescence for High Performance OLEDs

“…The two compounds exhibited main absorption peaks at 317 and 291 nm in toluene solution, originated from the π–π* transitions of conjugated molecular backbones, respectively. [ 54 ] Besides, BPI‐PhPXZ and BPI‐PhDMAC exhibited weak absorption bands spanning the scopes of 370–450 nm, ascribed to the ICT between the acceptor and donor units. [ 55 ] The FL spectra of BPI‐PhPXZ and BPI‐PhDMAC in toluene showed weak emission peaks at 551 and 504 nm with low PLQY values (8.3% for BPI‐PhPXZ and 28.1% for BPI‐PhDMAC).…”

Backbone‐Acceptor/Pendant‐Donor Strategy for Efficient Thermally Activated Delayed Fluorescence Conjugated Polymers with External Quantum Efficiency Close to 25% and Emission Peak at 608 nm

“…[ 15 ] To realize the redshifted emission in the TADF small molecule, extended conjugation system, similar to the traditional fluorescent and phosphorescent materials, is a familiar design method, as shown in Scheme a. [ 16,17 ] Strong donor and acceptor, respectively, provide a shallow highest occupied molecular orbital (HOMO) and a deep lowest unoccupied molecular orbital (LUMO) owing to extended conjugation, leading to a narrow bandgap and thus producing a redshifted emission. [ 18 ] On the other hand, extended conjugation is always accompanied by a rigid molecular structure, which can help to suppress internal conversion and enhance photoluminescence quantum yield (PLQY).…”